Odd010004 Bgp Protocol Design Issue 1_0

7/27/2019 Odd010004 Bgp Protocol Design Issue 1_0

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HUAWEI TECHNOLOGIES CO., LTD. All rights reserved

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ODD010004 BGP

Protocol Design

ISSUE 1.0

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HUAWEI TECHNOLOGIES CO., LTD. Page 1All rights reserved

The border gateway protocol (BGP) has

been developed from BGP1 (initial version

in 1989) to BGP4. As the Internet expands,

BGP functions are increased andstrengthened from the basic routing among

autonomous systems to community,

reflection, confederation, and multi-protocol

extension. The current number of BGP

routes on the Internet has exceeded

100,000 entries. Such a wide deployment

proves that BGP has a powerful support forlarge and complicated networks.

http://www.pdffactory.com/http://www.pdffactory.com/


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Upon completion this course, you will be able to:

[ Understand the advanced applications of

BGP

[ Optimize the path selection and convergence

time of BGP

[ Understand the network planning of BGP



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Chapter 1Chapter 1 BGP TheoriesBGP Theories

Chapter 2 BGP Network PlanningChapter 2 BGP Network Planning

Chapter 3 BGP Performance OptimizationChapter 3 BGP Performance Optimization



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Chapter 1 BGP TheoriesChapter 1 BGP Theories

Section 1 BGP PrinciplesSection 1 BGP Principles

Section 2 BGP Path AttributeSection 2 BGP Path Attribute

Section 3 Path Selection ProcedureSection 3 Path Selection Procedure



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BGP Principle

l BGP is an external routing protocol for transmission of routing

information among ASs.

l BGP is a path vector protocol, which is a kind of distance vector

protocols.

[ Reliable route update mechanism

[ Rich metric measurement methods

[ Free from loops by protocol design

l Carrying attributes for routes

l Supporting classless inter-domain routing (CIDR)

l Rich route filter and routing policies

BGP OverviewBGP Overview



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BGP Principle

BGP Neighbor RelationBGP Neighbor Relation

EBGPEBGP

RTB

RTC

IBGP

RTA

RTD

RTE

AS 100

AS 200

AS 300



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BGP Principle

l When there are several paths, BGP Speaker selects only the optimal one.

l BGP Speaker notifies only the routes used by itself to neighbor entities.

l Once the connection is established, BGP Speaker notifies its all BGP routes

to new neighbor entities.

l BGP Speaker notifies routes obtained from EBGP to all its neighbor entities

(including EBGP and IBGP).

l BGP Speaker does not notify the routes obtained from IBGP to its IBGP

neighbor entities.

l Whether BGP Speaker notifies the routes obtained from IBGP to its EBGP

neighbor entities depends on the IGP and BGP synchronization.

BGP Route Notification PrinciplesBGP Route Notification Principles



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BGP!"#

l Origin

l AS-Path

l

Next Hopl MED (Multi Exit Discriminator)

l Local-Preference

l Atomic-Aggregate

l Aggregator

l Community

Common BGP AttributeCommon BGP Attribute

l Originator-ID

l Cluster-List

l Destination Pref (MCI)

l Advertiser (Baynet)

l Rcid-Path (Baynet)

l MP_Reach_NLRI

l MP_Unreach_NLRI

l Extended_Communities



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BGP Path Selection Procedure

l BGP skips a route if its next hop is unreachable.

l BGP firstly selects the route of highest Local-Preference.

l BGP selects routes originated by local routers (which have a same priority).

l BGP evaluates the AS path length and selects firstly the shortest path.

l BGP compares the attributes of Origin and selects the path of the lowest attribute

of Origin. The Origin attribute of IGP is lower than that of EGP, and that of EGPlower than Incomplete.

l BGP selects the route of smaller MED.

l BGP selects EBGP prior to than IBGP routes.

l BGP firstly selects the path with shortest IGP metric to the next BGP hop.

l BGP compares the Originator_ID (Router ID if Originator_ID is unavailable) and

selects the path of smaller Originator_ID.

l BGP compares the peer IP address and selects the path with smallest value of IP

address.



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Section 1 IBGP/EBGPSection 1 IBGP/EBGP Core DesignCore Design

Section 2 MultiSection 2 Multi--homing Load Balancehoming Load Balance

Section 3Section 3 BGP Route ReflectorBGP Route Reflector

Section 4 BGP ConfederationSection 4 BGP Confederation

Section 5 BGP Community AttributeSection 5 BGP Community Attribute



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Questions

l When is BGP needed?

l Is BGP suitable for the current network?

l What are BGP's advantages and disadvantages?



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BGP Network Planning

IBGP/EBGP Core DesignIBGP/EBGP Core Design

IBGP Core Design

EBGP Core Design

(I/E)BGP Core Design



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IBGP Core Design

l BGP selects the path by comparing the IGP metric to the

destination router.

l If the core IGP has several paths to the destination router, BGP

selects the one with the smallest IGP metric and keeps others

in BGP route table in form of copies.

l If the core IGP has several equivalent paths to a destination

router, these paths are used as entries and added to the

routing table for load sharing.

Path SelectionPath Selection



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IBGP Core Design

l IBGP path selection depends on the IGP metric.

l IBGP path convergence time depends on the IGP working mechanism.

Fault and RecoveryFault and Recovery

RT1RT1RT2RT2

RT3RT3 RT4RT4

RT5RT5

RT6RT6

RT7RT7

RT8RT8



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EBGP Core Design

l If there are no policies, EBGP path selection depends on the

two parameters:

[ AS-PATH length

[ Neighbor router ID




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EBGP Core Design

RT2RT2

10.1.2.110.1.2.1

RT3RT3

10.1.3.110.1.3.1

RT4RT4

10.1.4.110.1.4.1

RT5RT510.1.5.110.1.5.1

RT6RT6

10.1.0.110.1.0.1

AS65101AS65101

AS65102AS65102

AS65103AS65103

AS65104AS65104

Route ImportRoute Import

192.168.1.0/24192.168.1.0/24



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EBGP Core Design

l Because physical topology and link bandwidth are invisible to BGP path

selection, a suboptimum route may be selected.

l The path selection can be changed by such policies as MED, Local_Pref,

and AS-PATH list. (Before changing default BGP attributes, you must

completely understand their meanings to avoid side effects.)


RT3192.168.1.0/24R6

RT3192.168.1.0/24R5

192.168.1.0/24R4

RT3192.168.1.0/24R2

Next HopDestination Address/MaskRouter

RT6RT6



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EBGP Core Design

l After disconnection of a BGP session, the update procedure

takes place one hop after another, till all BGPs in all affected

BGP autonomous systems announce that the all routers are

updated.

l The IGP that is unable to respond quickly to network changes

is a little slower in re-convergence. The re-convergence time is

accumulative.




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RT1RT1

RT3RT3

RT4RT4 RT5RT5RT7RT7

RT8RT8

RT9RT9

RT10RT10RT2RT2

RT3RT3 RT6RT6

AS65101AS65101

AS65102AS65102

RT11RT11AS65104AS65104

AS65103AS65103

192.168.2.0/24192.168.2.0/24

192.168.1.0/24192.168.1.0/24

192.168.4.0/24192.168.4.0/24

192.168.4.0/24192.168.4.0/24

AS65100AS65100



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RT1RT1

RT3RT3

RT4RT4 RT5RT5RT7RT7

RT8RT8

RT9RT9

RT10RT10RT2RT2

RT3RT3 RT6RT6

AS65101AS65101

AS65102AS65102

RT11RT11AS65104AS65104

AS65103AS65103

AS65100AS65100



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RT1RT1

RT3RT3

RT4RT4 RT5RT5RT7RT7

RT8RT8

RT9RT9

RT10RT10RT2RT2

RT3RT3 RT6RT6

AS65101AS65101

AS65102AS65102

RT11RT11AS65104AS65104

AS65103AS65103

AS65100AS65100



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MultiMulti--HomingHoming

Single Homing End Network

Multi-Homing End Network

Standard Multi-Homing Network



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Single Homing End Network

Disadvantages Several connections

are terminated on a

same device.

Several static routes

are needed.

Only link failure can

be avoided. Device

failure cannot be

avoided.

AS65101

AS65102

Advantages Cost-effective

BGP replaced by

static routing or

default routing



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Multi-Homing End Network ! Single Border Router

Disadvantages

Advantages

Several

connections are

terminated on a

same device.

Only link failure

can be avoided.

Device failure

cannot be avoided.

AS65101

AS65102

Private AS can be

used.

Outgoing traffic

can be better

controlled for loadbalance by link

capacity.



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Multi-Homing End Network ! Several Border Routers

Disadvantages

Advantages

Core router must

run IBGP session

and core IGP

session.

AS65101

AS65102

Private AS can be

used.

Outgoing traffic

can be better

controlled for load

balance by link

capacity.



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Standard Multi-Homing Main Network ! Single

Border Router

AS65101

AS65102

AS65103

Single Border RouterSingle Border Router



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Standard Multi-Homing Main Network ! Several

Border Routers

AS65101

AS65102

AS65103

Several Border RoutersSeveral Border Routers



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Load BalanceLoad Balance

Incoming Load Balance

Outgoing Load Balance

Multiple Sessions of A Peer



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Incoming Load Balance

l Multi-homing to a same upstream autonomous system

[ When the upstream system provides two link, the address

space can be reasonably designed.

l Multi-homing to two different upstream autonomous system

[ Monitor link and occupancy

[ Achieve load balance by adding AS-PATH attribute length

(an AS number added in one time)

Incoming Load BalanceIncoming Load Balance



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Outgoing Load Balance

l Multi-homing to a same upstream autonomous system

[ Achieve load balance by default routing

l Multi-homing to two different upstream autonomous systems

[ Achieve load balance by requesting some available routes

that are completely reachable

l If load balance still cannot be achieved, complete Internet

routes are requested and incoming filtering policies are

implemented to generate the partial routing table of the router

itself.

Outgoing Load BalanceOutgoing Load Balance



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l Loopback addresses are used to set up EBGP sessions. The

equivalent routes to the peer loopback addresses are iterated for

load balance.

l Because the default TTL of EBGP is 1, both sides need change

the TTL value by the command ebgp-max-hop.

AS65101 AS65102

LoopbackLoopback LoopbackLoopback

EBGP MultiEBGP Multi--Path SolutionPath Solution



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EBGP MultiEBGP Multi--Path SolutionPath Solution

l Every link in the autonomous system sets up the EBGP session.

l Because BGP supports only one equivalent path by default, both

sides need set the number of maximum equivalent paths by the

command maximum load-balancing.

AS65101 AS65102



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Reflector Notification

Principle

Router ReflectionRouter Reflection

Reflector Cluster

Loop Prevention Mechanism

Reflector Design Principle



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Reflector Notification Principle

l Only the known optimal path is notified or reflected.

l Notification is always made to EBGP peer.

l Clients observe the conventional IBGP loop prevention rules when

notifying the routes.

l Other rules are observed if the notifications are made to IBGP peer, client,

or non-clients. When the notification is made to IBGP peer, these rulesdepend on where these routes are learned from.

l If the reflector learns the routes from EBGP peer, it notifies to all its clients

and non-clients.

l If a route reaches the reflector through a non-client IBGP peer, the

reflector reflects this route to all its clients.

l If a route reaches the reflector through a client, the reflector reflects this

route to all other clients and non-clients.



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Reflector Clusterl Several reflectors serve one or several clients to provide redundancy.

l A 4-byte CLUSTER_ID identifies a cluster.

l The reflector receiving the information will ignore the route notification

that carries the same CLUSTER_ID.

l A cluster may include one or several reflectors.

l A client may belong to several clusters.

Cluster

10.1.1.1

Cluster

10.1.1.2

Cluster

10.1.1.3

Reflector

RT3

Reflector

RT4

Reflector

RT6

Reflector

RT5

Client

RT7

Client

RT1

ClientRT2



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Loop Prevention Mechanism

l To prevent loop caused by reflector during route update, two BGP path attributes areintroduced:

[ Originator_ID

" Originator_ID is created by the first reflector and will not be changed by

subsequent reflectors.

" It is a 32-bit value, and should be received from only IBGP.

" It is the Router ID of:

# The BGP announcer of local AS originated routes.

# The border router of a same AS if the route is learned from EBGP.

[ Cluster_List

"

Cluster_List records all Cluster_ID passed by and puts the latest Cluster_IDon the top of the Cluster_List.

" The reflector identifies loops by Cluster_List.



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Layered Router ReflectionLayered Router Reflection

Layer 2 Reflector

Layer 1 Reflector/Client

Client



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l Keep logical and physical topologies match to ensure redundancy,

optimize path, and prevent loops of route selection.

l Use comparable metric for route selection to avoid route oscillation

during convergence.

l Set proper intra- and inter-cluster IGP metrics to prevent route

oscillation during convergence.

l Apply cluster division techniques correctly to enforce the reflector

redundancy.

l Modify the path carefully if putting the reflector to the forwarding path.

l Use peer group to reduce convergence time if there are reflectors.



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ConfederationConfederation

Confederation Principle

AS-PATH Special Treatment

Comparison Between

Confederation and Reflector

Confederation Design

Principles



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Confederation Principles

l Confederation Concepts

[ A large AS is divided into several small ASs for the IBGP

full mesh.

AS 100

AS 200

AS 65100 AS 65200

AS 65300

RT1 RT2

RT3 RT4

RT5



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AS-PATH Special Treatmentl Two new AS-PATH subtypes are introduced in confederation:

[ AS-CONFED-SEQUENCE

[ AS-CONFED-SET

AS 100AS 100

AS 200AS 200

AS 65100AS 65100 AS 65200AS 65200

AS 65300AS 65300

RT1RT1 RT2RT2

RT3RT3 RT4RT4

RT5RT5



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Confederation Design Principles

l Hub-and-Spoke Architecture

AS 100AS 100

AS 65100AS 65100

AS 65200AS 65200

RT5RT5

RT6RT6

RT4RT4

AS 65300AS 65300

AS 65000AS 65000

RT1RT1

RT2RT2

RT3RT3



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l Improper setting of IGP metric in confederation will cause route

oscillation.

AS 103AS 103

AS 200AS 200

AS 65100AS 65100

AS 65200AS 65200

RT3RT3

RT5RT5

RT4RT4

RT6RT6

RT3RT3

AS 102AS 102

RT2RT2

AS 101AS 101

RT1RT1

Route ImportRoute Import

192.168.1.0/24192.168.1.0/24

1010

1010

1010

1010

1111

2020

1515

MED:10MED:10

MED:5MED:5

MED:6MED:6



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(65200) 103 101

103 101

102 101

AS-PATH

RT6

RT4

RT3

BGP Next Hop

1163

2052

15101

IGP MetricMEDPath

RT5 Routing TableRT5 Routing Table

l Solution: Proper setting of IGP metric in confederation

[ Enable compare-different-as-med.

[ Reset MED to 0.[ Set IGP metric properly.



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Comparison Between Confederation and Reflector

In face, AS combination is irrelevant with IBGP expandability. It is mentioned herebecause it is one of confederation advantages. An AS can be combined with an existingconfederation by taking AS as a sub-AS of confederation.

AS combination

More experiences are drawn from route reflection because more ISPs have deployedroute reflection rather than confederation.

Deploymentexperience

Route reflection within AS needs a stand-alone IGP, while confederation supports astand-alone or separated IGP, which may be the biggest advantage of confederation overroute reflection. If the IGP meets its limit of expandability, or it is too large to process themanagement tasks, confederation can be used to reduce the size of IGP routing table.

IGPexpandability

All routers in confederation must support confederation configuration capability, becauseall routers need understand the AS-PATH attribute of the confederation. In thearchitecture of route reflection, the reflector need support only route reflection capability.In new design of cluster division, however, the client is also required to understand thereflector attributes.

Supportcapability

The transfer of route reflection is of low complexity, because the overall networkconfiguration seldom changes. However, transfer from IBGP to confederation needs greatchanges on configuration and network architecture.

Complexity ofconventionalIBGP transfer

Both provide control of route selection policies, but confederation has more flexibility.Policy Control

Both support multilevel to further enhance the expandability. Multilevel route reflectionstructure is supported. Confederation allows route reflection used in member ASs.

Multilevel

ComparisonReference

Factor



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BGP Community AttributeBGP Community Attribute

Concept of BGP

Community Attribute

Design of BGP Community

Attribute



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Concept of BGP Community Attribute

l A group of routes sharing the same features define a BGP

community attribute. Several communities can be applied to a

route. Each community is of 4-byte long. There are two types

of community attribute:

[ Well-known communities

" NO_EXPORT

" LOCAL_AS

" NO_ADVERTISE

[ Private communities

"As:number



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Design of BGP Community Attribute

l Local AS differentiates routes.

[ Cross-over route

[ Peer route

[ User route

[ISP ASN]:1000Cross-over route

[ISP ASN]:3000

[ISP ASN]:2000

Community attribute ID

User route

Peer route

Route Type

Example:Example:

Route Origin TracingRoute Origin Tracing



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l Operation on local priority attribute

l Control by route policies

Dynamic user policyDynamic user policy

[ISP ASN]:6060

[ISP ASN]:8080

[ISP ASN]:100100

[ISP ASN]:140

[ISP ASN]:120

Community attribute ID

140

120

Local Preference Attribute

Example:Example:



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l Community attributes can be flexibly used to make the AS

notify user routes to upstream peer.

l In general, two levels are defined:

[ Level 1 differentiates all routes of local AS.

[ Level 2 allows the lower-level AS to set AS-PATH attributes

accordingly or suppresses route notifications completely.

Control of upstream route notificationControl of upstream route notification



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Section 1Section 1 Mitigation of Network Fault AffectsMitigation of Network Fault Affects

Section 2 Optimization of Route UpdateSection 2 Optimization of Route Update

Section 3 Policy ControlSection 3 Policy Control



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BGP performance optimization

EBGP Port Sensitivity

IGP/BGP Convergence Time Increment

Mitigation of Network Fault AffectsMitigation of Network Fault Affects

BGP Graceful Restart (GR)



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EBGP Port Sensitivity

l Fast disconnect EBGP sessions before timer timeout.

l It is applicable to only EBGP peer. IBGP peer may bypass the invalid link

if it is faulty.

l It is globally enabled and, by default, activated.

l Advantages:

[ When the upstream EBGP peer has several EBGP sessions, EBGP

port sensitivity is very useful.

l Disadvantages:

[ Owing to jitter links, BGP session is hard to enter convergence status.

[ It is unworkable in case of EBGP multi-hop. The peer address must be

the same as its physical address.

EBGP Port SensitivityEBGP Port Sensitivity



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Traffic LossTraffic Loss

AS 200AS 200

AS 100AS 100 AS 300AS 300

RTARTA

RTBRTB

RTERTE RTDRTD

RTCRTC



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l The router that restarted just now need notify a message in

IGP that is should not be used as a cross-over router. This

means the restarted router must be the leaf node of the

shortest path tree. Only direct traffic to the router should be

sent to this router.

[ IS-IS Overload Bit

[ OSPF Maximum Metric

SolutionSolution



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l The IS-IS protocol provides a feature named overload bit.

Being an LSP value, the overload bit is firstly used as a signal

of notification in case of router fault. It ensures that this router

should not be included in the network topology as one with

cross-over capability.

l Routers with overload bit are still allowed for setup of BGP

sessions and BGP re-convergence. However, they cannot be

used as cross-over routers.

l After convergence of BGP sessions, the router sends an LSP

notifying that the overload bit is removed.

ISIS--IS Overload BitIS Overload Bit



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l OSPF sets metric in LSA to ensure the router will not be used

as a cross-over path.

l Routers with metric are still allowed for setup of BGP sessions

and BGP re-convergence. However, they cannot be used ascross-over routers.

l After convergence of BGP session, the LSA for the new router

and the whole network are re-sent.

OSPF Maximum MetricOSPF Maximum Metric



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BGP-GR (Graceful Restard)

l Preventing route oscillation caused by active/standby board

switchover

l Ensuring no message loss when new board re-collects route

information from BGP peer

BGPBGP--GR FunctionsGR Functions



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Route Update and Optimization

l How to deal with the instability during route notification?

l How to keep the affect of applying new policies to the minimum?

l Available methods for route optimization:

[ Route jitter attenuation

[ BGP route Keep-All-Routes and Route Refresh

[ Loop detection at transmission side

[ Outgoing route filtering (ORF)

Purpose of Route Update and OptimizationPurpose of Route Update and Optimization



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Route jitter attenuation

l Route jitter attenuation provides a mechanism to reduce the

router load caused by instable routes.

l It prevents continuous route jitter.

l It enhances route stability, but not sacrifices the convergence

time of routes with good performance.

Purpose of Route Jitter AttenuationPurpose of Route Jitter Attenuation


BGP K All R t


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BGP Keep-All-Routes

l When BGP policies are changed, the BGP sessions need be

reset to make the new policies effective. Resetting BGP

sessions will cause route disturbance and route jitter. Resetting

a large number of BGP peers may even trigger route jitter

attenuation.

l Save the BGP route update information from all peers (groups)

since the setup of BGP connection, even if the routes do not

meet the entry policies requirements.

l Trigger update of route information by proper inbound and

outbound policies.

BGP KeepBGP Keep--AllAll--RoutesRoutes


R t R f h


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Route Refresh

l Route refresh is the negotiation ability of BGP at the initial

session stage.

l It allows BGP routers to request remote peers to resend the

outbound information library of BGP neighbor routes.

l Route refresh is the replacement of BGP Keep-All-Routes.

Route RefreshRoute Refresh


L b k d t ti t t i i id


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Loopback detection at transmission side

l Concept: A route will be rejected if the loop detection mechanism

of the peer detects its own AS number in AS-PATH.

l Optimization idea: These routes are blocked from notification at

the earliest time to reduce the size of BGP update packets.

l Applicable range: It is applicable to only EBGP peers.

l Implementation: It is optimized by matching AS-PATH list with the

remote AS in AS-PATH attributes in the notified route information.

l

Note: In special case, the loop detection at transmission side isnot expected, such as MPLS VPN Hub-And-Spoke.

LoopbackLoopback DetectionDetection


Outbound Route Filtering (ORF)


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Outbound Route Filtering (ORF)

l ORF Concept:

[ ORF is a BGP feature that reduces the number of BGP

updates sent between peer routes to the minimum with the

routes transmitting and receiving capability of BGP.

[ Local routers initially send inbound routing policies to

remote peer routers, which use them as outbound policies.

l ORF Advantages:

[ Reducing the number of BGP updates sent between peer

routes to the minimum.

[ Sparing bandwidth and speeding convergence

ORF Concept and AdvantagesORF Concept and Advantages



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Policy Control


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Policy Control

l ip prefix-list

l as-path-list

l community-list

l route-policy

l filter-policy

Policy Control MethodsPolicy Control Methods


Policy Control


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Policy Control

l What is pseudo-AS number?

[ When receiving/sending AS-PATH from/to the neighbor of

local AS, BGP places pseudo-AS number before the real

AS number.

l When is pseudo-AS number needed?

[ Change of AS number

[ Device transfer or BGP session transfer

PseudoPseudo--AS NumberAS Number


Policy Control


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Policy Control

l Advantages of Pseudo-AS Number:

[ Preventing network destruction caused by AS number

change.

[ Providing continuous communication for AS during AStransfer.

l Disadvantages of Pseudo-AS Number:

[ Making AS-PATH longer.

[ Causing packet loss due to loop detection

Advantages and Disadvantages of PseudoAdvantages and Disadvantages of Pseudo--AS NumberAS Number



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Summary l Same policy tool of BGP at first and at

last

l Affecting BGP performance most

seriously

l Multi-homing and load balance



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Documents

Odd010004 Bgp Protocol Design Issue 1_0